In ultrahigh speed optical communications, such phenomena as chromatic dispersion, polarization mode dispersion and bandwidth limitation in transmission line optical fibers or components in use are significant limiting factors for transmission rates and transmission distances. “Chromatic Dispersion” (CD) refers to a phenomenon that light with different wavelengths travels through an optical fiber at different speeds (hereinafter what is merely called “dispersion” refers to chromatic dispersion). It is known that the optical spectrum of an optical signal modulated at a high speed contains different wavelength components and these components arrive at the receiving end at different times under the influence of chromatic dispersion, resulting in a large waveform distortion in the optical waveform after transmission. In order to avoid the influence of such chromatic dispersion, a technique called “chromatic dispersion compensation” is under consideration (hereinafter what is merely called “dispersion compensation” refers to chromatic dispersion compensation). Dispersion compensation is a method of preventing waveform distortion of received light in which an optical device having chromatic dispersion characteristics opposite to those of optical fibers used in a transmission line is placed in an optical transmitter or optical receiver to negate the chromatic dispersion characteristics of the optical fibers. Dispersion compensation techniques under consideration include the use of dispersion compensation fibers having chromatic dispersion opposite to that of the transmission line, optical interferometers, optical circuits, optical Fiber Bragg Gratings (FBG), optical transversal filters and so on. Another approach under consideration is a technique in which an electric compensation circuit such as an electric transversal filter is placed in an optical receiver for the purpose of waveform deterioration compensation.
Particularly it is known that when an optical signal with 10 Gbps or more is transmitted several hundred kilometers or more, the problem of change in the amount of chromatic dispersion is caused by optical fiber temperature change, and thus studies have been made on a variable dispersion compensation technique which varies the amount of compensation depending on the change. Variable dispersion compensators which are known to be used in this technique are, for example, those which vary the amount of chromatic dispersion by giving a temperature gradient or distortion to optical Fiber Bragg Grating or by changing the temperature or phase for an optical interferometer system. In the case of the above electric transversal filter, it is possible to provide variable compensation by varying the filter characteristics. This type of variable dispersion compensator is used to compensate for insufficiency of chromatic dispersion tolerance of a high-speed optical transmitter-receiver. For example, the maximum dispersion tolerance of a 40 Gbps transmitter or receiver is very small (80 ps/nm) and for widely used single-mode fibers (SMF), this tolerance level just covers 4 km. Hence, in transmission using a fixed dispersion compensation device, it would be necessary to replace the device with a device with a different compensation amount every 4 km transmission distance in order to make the total dispersion in the transmission line 80 ps/nm or less. This means a serious problem that many types of dispersion compensators must be used, resulting in increase in the compensator management and their maintenance cost and longer time required for the manufacture and installation of the compensators. On the other hand, it might become necessary to measure the amount of chromatic dispersion and the length of the transmission line with high accuracy or the user could not change the transmission path easily or many other problems might occur.
With this background, studies have been made on an automatic dispersion compensation technique in which a variable dispensation compensator is placed just before an optical receiver to detect the amount of deterioration of received waveform or transmission characteristics and vary the amount of chromatic dispersion automatically so as to optimize the received waveform. This technique makes it possible that even a high-speed optical transmitter-receiver works when the user connects the equipment, without taking chromatic dispersion in the transmission line into consideration, as in the conventional method; namely it realizes “plug & play.”
On the other hand, “Polarization Mode Dispersion” (PMD) refers to a phenomenon that optical signals on two principal axes (TE and TM) inside an optical fiber travel at different speeds. It is known that as a consequence, optical signals distributed to the two principal axes TE and TM arrive at the receiving end at different times, causing a large waveform distortion. In order to avoid such influence of polarization mode dispersion, a technique called PMD compensation has been studied. PMD compensation refers to a technique that a device having polarization mode dispersion opposite to that of the transmission line is inserted in the transmission line to prevent optical waveform distortion. Another approach under consideration is a technique in which an electric compensation circuit such as a transversal filter is placed in an optical receiver to compensate for PMD-induced waveform deterioration. Unlike chromatic dispersion, the PMD amount in an optical fiber transmission line is known to change momentarily according to the ambient temperature or input state of polarization; therefore, automatic PMD compensation which detects the amount of deterioration and optimizes compensation to minimize the deterioration is indispensable.
“Bandwidth limitation” refers to a phenomenon that particular frequency components such as an optical signal high frequency component is lost because of bandwidth limits on a multi-mode optical fiber used as a transmission line, a semiconductor laser or photodiode used for generating or receiving optical signals, IC or the like, and leads to received optical waveform deterioration in high speed optical transmission. As for bandwidth limitation, studies have been made on a technique in which a compensation circuit such as an optical or electric transversal filter is placed to compensate for the weakened high frequency component; however, since this phenomenon also largely depends on the mode of input into the optical fiber, optical fiber condition, transmission distance and the optical spectrum characteristics or modulation characteristics of light sources of individual optical transmitters, the amount of compensation cannot be predetermined and automatic compensation is indispensable in which the amount of deterioration should be detected and compensation should always be optimized to minimize the deterioration. This compensation is effective not only for bandwidth limitation but also for some deterioration induced by chromatic dispersion or polarization mode dispersion and initial waveform inter-symbol interference.
For automatic control by many variable optical/electric compensators used in optical fiber transmissions, some technique of detecting the amount of deterioration of waveform or transmission characteristics is needed. FIG. 2 shows the configuration of a conventional automatic chromatic dispersion compensator which uses a clock extraction/maximum control method as a typical method for waveform deterioration detection in variable dispersion compensation or PMD compensation.
An optical digital data signal which has deteriorated due to optical fiber chromatic dispersion or polarization mode dispersion in optical fiber transmission enters a conventional automatic chromatic dispersion compensator 102 through an input optical fiber 101. As it passes through a variable optical chromatic dispersion compensator 102, the optical signal is subjected to compensation for its deterioration induced by chromatic dispersion, before being outputted through an output optical fiber 105. When the compensator 102 is a PMD compensator, it is also possible to realize a variable PMD compensator with an almost equivalent configuration. Part of the compensated optical signal is branched by an optical splitter 104 and leaded to an optical detector 106 and converted into an electric signal. The electric signal is rectified by a rectifier 121 and the clock component is extracted from the received signal by filtering the output signal by a bandpass filter 122 whose transmission center bandpass is equal to the bit rate. Since the intensity of this clock signal is almost proportional to the eye-opening of the received waveform, a control signal 103 obtained from a maximum control circuit 123 is sent to the variable optical chromatic dispersion compensator 102 to change the amount of chromatic dispersion and perform maximum control so as to maximize the clock signal, so that the waveform deterioration is maintained minimum at all times.
Control of a variable chromatic dispersion compensator by clock extraction as mentioned above has been reported, for example, in “Extracted-Clock Power Level Monitoring Scheme for Automatic Dispersion Equalization in High-Speed Optical Transmission Systems” (IEICE Trans. Commun., Vol; E84-B, No. 11 Nov. 2001). In this paper, FIG. 6 shows the relation between clock component intensity and transmission line chromatic dispersion in 20 Gbps NRZ (Non Return to Zero)/RZ (Return to Zero) systems. For example, in case of the NRZ signal indicated by solid line in FIG. 6B, the clock signal intensity (vertical axis) is maximum at a chromatic dispersion of −150 ps/nm (horizontal axis) and the waveform at this point is the best. When the amount of chromatic dispersion is +50 ps/nm to −350 ps/nm, namely in the range of about 400 ps/nm, the clock intensity curve is of the single-peak type, or has an upward peak at the dispersion of −150 ps/nm, which means that the best waveform is always obtained by controlling compensation by the variable chromatic dispersion compensator so as to maximize the clock intensity.
Non-patent Document 1
“Extracted-Clock Power Level Monitoring Scheme for Automatic Dispersion Equalization in High-Speed Optical Transmission Systems” (IEICE Trans. Commun., Vol; E84-B, No. 11 Nov. 2001)